Yeah, I think that's primarily what bothers me about the current implementation (aside from the few places where it's triggered too easily). Getting harried sucks, and isn't fun. But it's still a reasonable challenge to try to zerg through, and avoid getting in trouble by being harried. Means learning when to fight, and when to run, instead of just run, run, run.
I think maybe a better thing, that would be relatively easy, would be to give monsters a speed boost, and a reach boost, when DA goes up, so that running away doesn't prevent being attacked. This would be a sort of workaround for the lousy AI that has trouble hitting targets that are running away.
But I think the principle of "too much zerging -> something happens to reign you in" is sound. Slowing movement rate just isn't fun. On the other hand, the boss buffs are reasonable.